The present invention concerns a metering device for metal, especially magnesium.
Various metering devices are available for feeding metal to automatic casting machines. They can be based on centrifugal forces, mechanical, electromechanical, gravimetric forces or gas pressure. Of these, pumps based on gas pressure and gravimetric forces (siphon) are used most commonly in magnesium foundries today. Rapid cycle times and the need for exact metering of the quantity of metal set high requirements for the metering system.
Standard centrifugal pumps and piston pumps have parts which are moved in the liquid metal. This gives rise to movement of the metal melt with the consequent formation of oxides. The pump inlet is usually located close to the base of the crucible with a danger of pumping contaminated metal. The pump parts which move in the liquid metal can suffer accelerated wear, which leads to imprecise measurements and high maintenance costs.
A siphon system is probably the metering system which is used most commonly for magnesium today. The inlet end, which is located in the liquid metal, is fitted with a valve which is opened and closed by a pneumatic cylinder. When the siphon is to be used the pipe is evacuated, filled with metal and the valve is closed. In the start position the discharge end must be lower than the level of metal in the furnace. For safety reasons the discharge end of the pipe is raised between each metering so that the level of metal in the discharge end equals or slightly exceeds the level of the metal in the furnace. This causes movement in the melt so that the surface film caused by the use of protective gas must be replaced. With this metering arrangement there have also been problems with leaky valves which produce imprecise weights for small shot quantities. Nor is it possible to alter the metering speed as the speed is dependent on the angle of incline of the pipe.